Einstein called it "spooky action at a distance" but today quantum entanglement is poised to revolutionize technology from computers to cryptography. Physicists have gradually become convinced that the phenomenon is real - two subatomic particles that mirror changes in each other instantaneously over any distance - but a few doubts remain. Nova follows a ground-breaking experiment in the Canary Islands to use quasars at opposite ends of the universe to once and for all settle the few remaining questions.